Dna sequences coding for a polyol carrier and use thereof, in particular for preparing transgenic plants

ABSTRACT

The invention concerns the use of a DNA sequence coding for a polyol carrier, in plants and fungi, such as polyols having a main chain containing 5 to 8 carbon atoms, in particular 5 to 7 carbon atoms, more preferably 6 carbon atoms, the polyols being advantageously selected among mannitol, sorbitol, dulcitol, galactitol, inositol, myo-inositol, ribitol and xylitol, and being preferably mannitol, for preparing transgenic plants.

The invention relates to DNA sequences coding for a polyol carrier and their use, in particular for the preparation of transgenic plants.

The plants are capable of synthesizing, via photosynthesis, primary compounds such as glucides by using light energy. Only certain organs of the plant, mainly the adult leaves, are capable of manufacturing and exporting the glucides towards the storage organs, such as the tubers, the seeds and the fruits, used in human and animal foodstuffs.

In the majority of plants, the main glucide transported is saccharose, but in a large number of plants, other compounds are also transported such as polyols of which mannitol is an example.

Polyols are, like saccharose, primary products of photosynthesis. It has furthermore been estimated that approximately 30% of the global production of primary carbon was used for the synthesis of polyols.

Polyols, cyclic or non-cyclic, are very widespread in plants; they are low-molecular weight, very soluble and non-reducing compounds. The three non-cyclic polyols (alditols) which are most widespread amongst the Angiosperms are galactitol, sorbitol and mannitol. Sorbitol is the main photosynthetic product in several species of Rosaceae such as the apple, the pear, the peach and the plum.

Mannitol, the most widespread of the alditols, is present in more than 100 species of higher plants, in particular in the Rubiaceae (coffee), the Oleaceae (privet, ash, olive) and the Apiaceae (celery, carrot, parsley) (Lewis, 1984). It is produced in the mesophyll cells (cells containing chlorophyll). To circulate, it must re-enter the sieve tubes (veins). However, there is no continuity between the mesophyll cells and the sieve tubes: a mannitol carrier is therefore needed. In this way, the mannitol leaves the mesophyll cells and uses the carrier to enter the sieve tubes.

The compounds synthesized in the adult leaves are transported towards the storage organs and cross a certain number of membranes using the specialized proteins that are the carriers. These carriers play a considerable role in the plant as they are essential for its growth.

The existence of a mannitol carrier in a plant such as celery has been shown by different biochemical experiments (Salmon et al., 1995). This publication has shown that there was a mannitol carrier in celery and that the expression of this carrier was very sizeable in the tissues of the phloem. However, nothing is said as to the identification of the mannitol carrier.

If numerous carriers of sugars, such as saccharose and the hexoses have been cloned during the course of the last few years, none of them is capable of transporting polyol.

At present, no carrier of linear polyol has been identified in a living organism. In bacteria, a multienzymatic system capable of both transporting and phosphorylating mannitol has been described (Boer et al., 1994). However, such systems have never been described in the higher organisms.

A subject of the invention is carriers of polyols in plants and fungi, and their DNA sequences.

A subject of the invention is also the use of DNA sequences of a polyol carrier for obtaining transgenic plants.

A subject of the invention is also the use of DNA sequences of a polyol carrier, in particular within the scope of obtaining plants resistant to pathogens or plants resistant to saline stress.

A subject of the invention is also the use of DNA sequences of a polyol carrier within the scope of a method of screening genetically modified plants.

The invention relates to the use of a DNA sequence coding for a linear polyol carrier, in plants and fungi,

-   -   such as polyols having a main chain containing 5 to 8 carbon         atoms, in particular 5 to 7 carbon atoms, in particular 6 carbon         atoms, these polyols being advantageously chosen from mannitol,         sorbitol, dulcitol, galactitol, inositol, ribitol and xylitol,         and being in particular mannitol, for the preparation of         transgenic plants.

In the expression “plants and fungi”, are included algae, mosses (Bryophytes), ferns (Pteridophytes), higher plants (Gymnosperms and Angiosperms) and fungi.

It can be recalled that, by definition, a polyol is a “polyalcohol” containing as many alcohol functions as carbon atoms. It can also be specified that the terms polyol, polyalcohol and alcohol sugar are equivalents.

According to an advantageous embodiment, the invention relates to the use, for the preparation of transgenic plants, of a DNA sequence chosen from one of the following sequences: SEQ ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9 and SEQ ID NO: 10.

SEQ ID NO: 1 is a new nucleic acid sequence identified in celery (Apium graveolens L.), coding for a mannitol carrier.

SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9 and SEQ ID NO: 10 are sequences of nucleic acids coding for proteins, the functions of which were unknown until now.

SEQ ID NO: 3 (Beet 1) and SEQ ID NO: 4 (Beet 2) originate from the Beetroot (Beta vulgaris).

SEQ ID NO: 5 (Pst 1), SEQ ID NO: 6 (Pst 2), SEQ ID NO: 7 (Pst 3), SEQ ID NO: 8 (Pst 4) and SEQ ID NO: 9 (Pst 5) originate from Arabidopsis thaliana.

SEQ ID NO: 10 (Bs) originates from Bacillus subtilis.

The invention also relates to a new protein, characterized by the fact that it comprises or is constituted by:

-   -   sequence SEQ ID NO: 2,     -   or any sequence derived from SEQ ID NO: 2, in particular by         substitution, suppression or addition of one or more amino         acids, having the property of transporting linear polyols in         plants and fungi,     -   such as polyols having a main chain containing 5 to 8 carbon         atoms, in particular 5 to 7 carbon atoms, in particular 6 carbon         atoms, these polyols being advantageously chosen from mannitol,         sorbitol, dulcitol, galactitol, inositol, ribitol and xylitol,         and being in particular mannitol,         -   any homologous sequence of SEQ ID NO: 2, preferably having a             homology of at least approximately 50% with sequence SEQ ID             NO: 2 and possessing the property of transporting, in plants             and fungi, polyols as defined above,         -   or any fragment of one of the sequences defined above, on             the condition that it has the property of transporting, in             plants and fungi, polyols as defined above, in particular             any fragment being constituted of at least approximately 10             amino acids adjacent in the sequence SEQ ID NO: 2.

The property of transporting polyols presented by a polyol carrier can be verified by one or other of the following tests:

-   -   the use of S. cerevisiae yeast or     -   the use of purified plasmic membrane of phloem vesicles.

The use of the yeast Saccharomyces cerevisiae (Noiraud et al., 2000) comprises the transformation of yeasts with the nucleotide sequence to be tested, these yeasts are capable of growing on said polyol. To verify that the polyol is transported in these yeasts, radioactively labelled polyol can be used. For each experiment, a control is perfected with a strain of yeast incapable of growing on the polyol, and which does not transport said polyol.

The test using a purified plasmic membrane from phloem vesicles is that described by Salmon et al. (1995).

According to an advantageous embodiment of the invention, the protein of the invention, as defined above, is characterized in that it is constituted by the sequence SEQ ID NO: 2.

The invention also relates to the protein fragments as defined above, chosen from the following sequences:

-   -   Ala Cys Ala Leu Leu Ala Ser Met Asn Ser Ile Leu Leu Gly Tyr Asp         Thr Gly Val Leu Ser Gly Ala Ser Ile (SEQ ID NO: 11) delimited         from the amino acid in position (26) to the amino acid in         position (50) of the sequence SEQ ID NO: 2,     -   Gln Ile Glu Ile Ile Ile Gly Ile Ile Asn Ile Tyr Ser Leu Leu Gly         Ser Ala Ile Ala Gly (SEQ ID NO: 12) delimited from the amino         acid in position (62) to the amino acid in position (82) of the         sequence SEQ ID NO: 2,     -   Tyr Thr Met Val Leu Ala Gly Ile Ile Phe Phe Leu Gly Ala Ile Phe         Met Gly Leu Ala (SEQ ID NO: 13) delimited from the amino acid in         position (92) to the amino acid in position (111) of the         sequence SEQ ID NO: 2,     -   Phe Leu Met Phe Gly Arg Phe Val Ala Gly Ile Gly Val Gly Tyr Ala         Met Met Ile Ala Pro Val Tyr Thr Ala (SEQ ID NO: 14) delimited         from the amino acid in position (116) to the amino acid in         position (140) of the sequence SEQ ID NO: 2,     -   Phe Leu Thr Ser Phe Pro Glu Val Phe Ile Asn Ser Gly Val Leu Leu         Gly Tyr Val Ser Asn Phe Ala Phe Ala (SEQ ID NO: 15) delimited         from the amino acid in position (150) to the amino acid in         position (174) of the sequence SEQ ID NO: 2,     -   Ile Met Leu Gly Ile Gly Ala Phe Pro Ser Val Ala Leu Ala Ile Ile         Val Leu Tyr Met (SEQ ID NO: 16) delimited from the amino acid in         position (184) to the amino acid in position (203) of the         sequence SEQ ID NO: 2,     -   Ala Ala Ile Thr Gly Ile Gly Ile His Phe Phe Gln Gln Ala Cys Gly         Ile Asp Ala Val Val Leu (SEQ ID NO: 17) delimited from the amino         acid in position (281) to the amino acid in position (302) of         the sequence SEQ ID NO: 2,     -   Leu Leu Ala Thr Ile Ala Val Gly Val Cys Lys Thr Val Phe Ile Leu         Ile Ser Thr Phe (SEQ ID NO: 18) delimited from the amino acid in         position (320) to the amino acid in position (339) of the         sequence SEQ ID NO: 2,     -   Leu Met Leu Thr Ser Met Gly Gly Met Val Ile Ala Leu Phe Val Leu         Ala Gly Ser Leu Thr Val (SEQ ID NO: 19) delimited from the amino         acid in position (349) to the amino acid in position (370) of         the sequence SEQ ID NO: 2,     -   Gly Gly Leu Ala Ile Phe Thr Val Tyr Ala Phe Val Ser Ile Phe Ser         Ser Gly Met Gly Pro Ile Ala Trp Val Tyr (SEQ ID NO: 20)         delimited from the amino acid in position (382) to the amino         acid in position (407) of the sequence SEQ ID NO: 2,     -   Cys Ser Ile Gly Val Ala Val Asn Arg Gly Met Ser Gly Ile Ile Gly         Met Thr Phe Ile Ser (SEQ ID NO: 21) delimited from the amino         acid in position (421) to the amino acid in position (441) of         the sequence SEQ ID NO: 2, and     -   Ala Phe Leu Leu Phe Ala Val Val Ala Ser Ile Gly Trp Val Phe Met         Tyr Thr Met Phe (SEQ ID NO: 22) delimited from the amino acid in         position (451) to the amino acid in position (470) of the         sequence SEQ ID NO: 2.

The invention also relates to a nucleotide sequence coding for a protein as defined above.

An advantageous DNA sequence of the invention comprises or is constituted by:

-   -   the nucleotide sequence SEQ ID NO: 1,     -   or any nucleotide sequence derived by degeneration of the         genetic code, of the sequence SEQ ID NO: 1 coding for a protein         represented by SEQ ID NO: 2,     -   or any nucleotide sequence derived, in particular by         substitution, suppression or addition of one or more         nucleotides, of the sequence SEQ ID NO: 1 coding for a protein         derived from SEQ ID NO: 2, as defined above,     -   or any homologous nucleotide sequence of SEQ ID NO: 1,         preferably having a homology of at least approximately 35% with         the sequence SEQ ID NO: 1 coding for a homologous protein of SEQ         ID NO: 2, as defined above,     -   or any fragment of the nucleotide sequence SEQ ID NO: 1 or of         the nucleotide sequences defined above, said fragment being         preferably constituted of at least approximately 30 nucleotides         adjacent in said sequence,     -   or any complementary nucleotide sequence of the above-mentioned         sequences or fragments,     -   or any nucleotide sequence capable of hybridizing in stringent         conditions with the complementary sequence of one of the         abovementioned sequences or fragments.

By stringent conditions of hybridization is understood:

-   -   temperature of hybridization: 65° C.,     -   hybridization medium: sodium phosphate buffer 250 mM, pH 7.2;         6.6% (w/v) of SDS; 1 mM EDTA; 1% (w/v) of bovine serum albumin,     -   washing temperature: 65° C.,     -   successive rinsing media:         -   2×SSC (1.75% NaCl; 0.88% sodium citrate), SDS 0.1%         -   1×SSC (0.875% NaCl; 0.44% sodium citrate), SDS 0.1%         -   0.5×SSC (0.44% NaCl; 0.22% sodium citrate), SDS 0.1%.

The invention also relates to the fragments of nucleotide sequences as defined above, chosen from the following sequences:

-   -   GCT TGT GCT CTT TTA GCT TCC ATG AAT TCC ATC TTA CTC GGC TAT GAC         ACC GGA GTG TTG AGT GGA GCA TCA ATA (SEQ ID NO: 23) delimited         from the nucleotide in position (92) to the nucleotide in         position (166) of the sequence SEQ ID NO: 1,     -   CAA ATC GAA ATA ATC ATC GGA ATC ATC AAC ATC TAC TCT CTT CTT GGT         TCG GCC ATA GCC GGA (SEQ ID NO: 24) delimited from the         nucleotide in position (200) to the nucleotide in position (262)         of the sequence SEQ ID NO: 1,     -   TAC ACC ATG GTA CTA GCT GGT ATC ATA TTT TTT CTA GGA GCC ATT TTC         ATG GGG CTT GCT (SEQ ID NO: 25) delimited from the nucleotide in         position (290) to the nucleotide in position (349) of the         sequence SEQ ID NO: 1,     -   TTT CTC ATG TTT GGT CGC TTT GTT GCT GGA ATT GGT GTC GGT TAT GCC         ATG ATG ATC GCT CCC GTC TAC ACT GCC (SEQ ID NO: 26) delimited         from the nucleotide in position (362) to the nucleotide in         position (436) of the sequence SEQ ID NO: 1,     -   TTC CTC ACT TCT TTT CCT GAG GTT TTC ATT AAT TCT GGT GTG TTG CTC         GGG TAT GTA TCC AAC TTT GCA TTT GCC (SEQ ID NO: 27) delimited         from the nucleotide in position (464) to the nucleotide in         position (538) of the sequence SEQ ID NO: 1,     -   ATT ATG CTG GGA ATT GGA GCA TTT CCT TCA GTT GCC TTG GCC ATA ATT         GTG TTA TAT ATG (SEQ ID NO: 28) delimited from the nucleotide in         position (566) to the nucleotide in position (625) of the         sequence SEQ ID NO: 1,     -   GCT GCA ATT ACG GGT ATT GGT ATT CAT TTC TTC CAA CAG GCT TGT GGT         ATT GAT GCT GTT GTT TTA (SEQ ID NO: 29) delimited from the         nucleotide in position (857) to the nucleotide in position (922)         of the sequence SEQ ID NO: 1,     -   CTC CTT GCG ACA ATT GCT GTT GGA GTC TGC AAA ACA GTC TTT ATT CTG         ATA TCA ACG TTT (SEQ ID NO: 30) delimited from the nucleotide in         position (974) to the nucleotide in position (1033) of the         sequence SEQ ID NO: 1,     -   CTG ATG CTA ACA AGT ATG GGG GGT ATG GTT ATT GCT CTA TTT GTA CTG         GCA GGC TCA TTG ACG GTT (SEQ ID NO: 31) delimited from the         nucleotide in position (1061) to the nucleotide in         position (1126) of the sequence SEQ ID NO: 1,     -   GGT GGT TTG GCA ATA TTT ACA GTG TAT GCT TTT GTG TCG ATA TTT TCA         AGT GGC ATG GGT CCA ATT GCT TGG GTC TAT (SEQ ID NO: 32)         delimited from the nucleotide in position (1160) to the         nucleotide in position (1237) of the sequence SEQ ID NO: 1,     -   TGT AGT ATC GGA GGT GCA GTT AAC CGT GGC ATG AGT GGC ATA ATT GGA         ATG ACA TTT ATA TCG (SEQ ID NO: 33) delimited from the         nucleotide in position (1277) to the nucleotide in         position (1339) of the sequence SEQ ID NO: 1,     -   GCA TTC CTT TTA TTT GCT GTG GTT GCA TCT ATC GGA TGG GTC TTT ATG         TAC ACA ATG TTC (SEQ ID NO: 34) delimited from the nucleotide in         position (1367) to the nucleotide in position (1426) of the         sequence SEQ ID NO: 1,

The nucleic acid sequence SEQ ID NO: 23 codes for the protein fragment SEQ ID NO: 11.

The nucleic acid sequence SEQ ID NO: 24 codes for the protein fragment SEQ ID NO: 12.

The nucleic acid sequence SEQ ID NO: 25 codes for the protein fragment SEQ ID NO: 13.

The nucleic acid sequence SEQ ID NO: 26 codes for the protein fragment SEQ ID NO: 14.

The nucleic acid sequence SEQ ID NO: 27 codes for the protein fragment SEQ ID NO: 15.

The nucleic acid sequence SEQ ID NO: 28 codes for the protein fragment SEQ ID NO: 16.

The nucleic acid sequence SEQ ID NO: 29 codes for the protein fragment SEQ ID NO: 17.

The nucleic acid sequence SEQ ID NO: 30 codes for the protein fragment SEQ ID NO: 18.

The nucleic acid sequence SEQ ID NO: 31 codes for the protein fragment SEQ ID NO: 19.

The nucleic acid sequence SEQ ID NO: 32 codes for the protein fragment SEQ ID NO: 20.

The nucleic acid sequence SEQ ID NO: 33 codes for the protein fragment SEQ ID NO: 21.

The nucleic acid sequence SEQ ID NO: 34 codes for the protein fragment SEQ ID NO: 22.

The invention also relates to a recombinant vector, in particular plasmid, cosmid, phage or virus DNA, containing a nucleotide sequence as mentioned above.

The invention also relates to a recombinant vector as defined above, containing the elements necessary for expression in a host cell of polypeptides coded by the nucleic acids as defined above, inserted into said vector.

According to an advantageous embodiment of the invention, the recombinant vector defined above contains in particular a promoter recognized by the RNA polymerase of the host cell, in particular an inducible promoter and optionally a transcription or termination sequence, and optionally a signal and/or anchoring sequence.

According to another advantageous embodiment of the invention, the recombinant vector, such as defined above, contains the elements which allow the expression of a nucleotide sequence, as defined above, as a mature protein or fusion protein.

The invention also relates to a host cell, chosen in particular from bacteria, viruses, yeasts, fungi, plants or the cells of mammals, said host cell being transformed, in particular using a recombinant vector as defined above.

According to an advantageous embodiment of the invention, the host cell, as defined above, contains the regulation elements allowing the expression of the nucleotide sequence as defined above.

The invention also relates to the product of the expression of a nucleic acid expressed by a host cell transformed as defined above.

The invention also relates to an antibody characterized in that it is directed in a specific manner against a protein of the invention.

The invention is not limited to polyclonal antibodies; the invention also relates to any monoclonal antibody produced by any hybridoma capable of being formed according to standard methods starting from, on the one hand, animal, in particular mouse or rat, spleen cells, the cells of the animal being immunized against the protein of the invention, and on the other hand cells of a cell line of myeloma, said hybridoma being capable of being chosen according to the capacity of the cell line to produce monoclonal antibodies recognizing the protein used beforehand for the immunization of the animals.

The invention also relates to a nucleotide probe capable of hybridizing with any one of the nucleic sequences of the invention.

The invention also relates to the antisense oligonucleotides or antisense messenger RNA, derived from the nucleotide sequences as defined above.

By modification of the expression of the mannitol carrier, using antisense oligonucleotides, it can then be determined if a reduction of the expression of the mannitol carrier has the result of reducing tolerance to saline stress.

The invention also relates to plant cells containing in their genome a nucleotide sequence as defined above.

The invention also relates to the transgenic plants, parts of plants, plant seeds or plant propagation material containing cells such as defined above.

The invention relates in particular to transgenic plants which, in their native state, do not contain or express the gene of the mannitol carrier, in the genome of which said nucleotide sequence is introduced.

The invention relates in particular to the transgenic plants which, in their native state, contain or express the gene of the mannitol carrier, in the genome of which said nucleotide sequence is introduced.

The invention also relates to a process for the preparation of a recombinant protein as defined above, comprising the following stages:

-   -   Culture in an appropriate medium of a host cell which has been         transformed beforehand by an appropriate vector containing a         nucleic acid of the invention, and     -   Recovery of the protein produced by the abovementioned host cell         transformed from the abovementioned culture medium or from the         host cell.

For example, a process for the preparation of a transgenic celery as defined above, comprises the following stages:

-   -   inoculation of the celery tissues,     -   coculture of the celery segments and of A. tumefaciens bacteria,     -   elimination of the A. tumefaciens bacteria,     -   regeneration of the transformed celery plants (Nadel et. al.,         1989).

The nucleotide sequences of the invention can be introduced into plasmids and be combined with regulation elements for expression in eukaryotic cells. These regulation elements are on the one hand transcription promoters and on the other hand transcription terminators. With the nucleotide sequences of the invention contained in the plasmids, the eukaryotic cells can be transformed with the intention of expressing a translatable mRNA which makes the synthesis of a polyol carrier in the cells possible or with the intention of expressing a non-translatable mRNA, which prevents the synthesis of a polyol carrier endogenous in the cells.

The processes of genetic modification of dicotyledons and monocotyledons are already known (Gasser et al., 1989). For expression in plants, the nucleotide sequences of the invention must be conjugated with transcription regulation elements. Such elements, called promoters, are already known (EP 375091).

In addition, coding regions with the termination signals of the transcription with which they can be correctly transcribed must be provided. Such elements are also described (Gielen et al., 1989). The initiation region of the transcription can be native and/or homologous as well as foreign and/or heterologous to the plant host. If desired, the termination regions are interchangeable amongst themselves. The DNA sequence of the initiation and termination regions of the transcription can be prepared synthetically or obtained naturally, or obtained from a mixture of natural or synthetic DNA constituents. To introduce foreign genes in higher plants, a large number of cloning vectors are available which include a replication signal for E. coli and a marker which allows selection of the transformed cells.

For the introduction of the nucleotide sequences of the invention into a plant host cell, in addition to transformation using Agrobacteria, there are many other techniques. These techniques include the fusion of protoplasts, the microinjection of DNA and electroporation, as well as ballistic methods and viral infection. Starting from the transformed plant material, whole plants can be regenerated in a suitable medium, containing antibiotics or biocides for the selection. The resulting plants can then be tested for the presence of the DNA introduced. There is no particular requirement for the plasmids regarding the injection and the electroporation. Single plasmids can be used such as the pUC derivatives. The presence of a marker gene is necessary for the regeneration of whole plants from such transformed cells. The transformed cells develop in the plants in the usual manner (McCormick et al., 1986). These plants can develop normally and be crossed with plants which possess the same transformed genes or different genes. The resulting hybrids have the corresponding phenotypic properties.

The DNA sequences of the invention can also be introduced into plasmids and be combined with regulation elements for an expression in prokaryotic cells.

The DNA sequences of the invention can also be introduced into plasmids which allow a mutagenesis or a sequence modification by means of a recombination of DNA sequences in prokaryotic or eukaryotic systems.

The transgenic plants of the invention are in particular characterized by an increase of the capacity to transport a polyol of the invention and to accumulate it in the organs from which it is extracted. They can be used to direct the flow of said polyol with the aid of said carrier towards the organs which accumulate little salt, thus facilitating extraction.

The invention also relates to a process of screening genetically modified plants with at least one nucleotide sequence of interest which comprises the following stages:

-   -   the transformation of plant cells with a vector containing an         insertion sequence, said insertion sequence comprising the         nucleotide sequence of interest and a nucleotide sequence coding         for a polyol carrier as defined above,     -   the culture of the cells thus transformed on a medium containing         said polyol as an only source of carbon, to obtain transgenic         plants or fragments of transgenic plants containing said         insertion sequence.

This process relates to plants not synthesizing polyol or plants which synthesize it.

It concerns, more particularly, the transformation of fragments or of plant cells with a nucleotide sequence coding for a polyol carrier, in particular mannitol. The screening is then carried out on a medium containing said polyol as the only source of carbon. The plants expressing the polyol carrier thus have an advantage in growth over the non-transformed plants. At this stage, it can be supposed that any plant is capable of using said polyol as a source of carbon. However, it can prove necessary to do a co-transformation with a gene coding a protein capable of degrading said polyol. The use of an active promoter only in the initial phases of regeneration or inducible by a simple compound makes it possible to restrict the expression of the polyol carrier to the selection phases.

The invention therefore concerns a simple selection system based on a plant gene which is no longer necessary once the selection is finished and on the use of a natural product as a selection agent. This system avoids having to resort to the use of products likely to be toxic, such as antibiotics.

The invention also relates to a process for obtaining transgenic plants resistant to pathogens, which comprises the following stages:

-   -   transformation of plant cells with a nucleotide sequence coding         for a polyol carrier as defined above,     -   culture of the thus transformed cells to obtain transgenic         plants or fragments of transgenic plants.

This process relates to the transformation of plants not synthesizing polyol or plants which synthesize it, with a nucleotide sequence of a polyol carrier, in particular mannitol, placed either under the control of a ubiquist promoter (type CaMV 35S) or under the control of an inducible promoter in response to the attack of the pathogen. The usefulness resides in the fact that the plant, in transporting more polyol, produced by the pathogen, towards its own cells, suppresses one of the means of defence put in place by the pathogen to fight against the activated oxygen released by the plant in response to this attack.

In order to increase the effectiveness of the process, the expression of the polyol carrier can be conjugated with an enzyme degrading said polyol.

The invention relates to a process for obtaining transgenic plants resistant to saline stress, which comprises the following stages:

-   -   transformation of plant cells with a nucleotide sequence coding         for a polyol carrier as defined above,     -   culture of the cells thus transformed to obtain transgenic         plants or fragments of transgenic plants.

This process relates to the transformation of plants not synthesizing polyol or plants which synthesize it with a nucleotide sequence coding for a polyol carrier placed under the control of a phloem-specific promoter (or of the promoter of the polyol carrier). If the plant synthesizes said polyol, the increase of the transport of said polyol could lead to an accumulated tolerance to saline stress. In the opposite case, it is also advisable to introduce genes allowing the synthesis of said polyol, but limiting this synthesis to the leaves in order to avoid harmful effects on the growth of the plant.

DESCRIPTION OF THE FIGURES

FIG. 1 represents the growth test of the yeast MaDH4 expressing the proteic sequence AgMaT1. The cDNA of AgMaT1, under the control of the promoter ADH1, was introduced in the cells of the MaDH4 strain, and the growth of the transformed cells on mannitol was studied. The transformed cells were grown on the SC (synthetic complete) liquid medium without tryptophan containing either 2% glucose (SC-glu) or 2% mannitol (SC-mann).

MaDH4-YEP112A1XE: MaDH4 containing the empty plasmid;

MaDH4-AgMaT1: MaDH4 containing the plasmid with the nucleic acid of AgMaT1.

The white squares correspond to MaDH4 yeasts transformed with the empty plasmid YEP112A1XE (defined hereafter) and grown on SC-glucose medium.

The black squares correspond to MaDH4 yeasts transformed with the empty plasmid YEP 112A1XE (defined hereafter) and grown on SC-mannitol medium.

The white circles correspond to MaDH4 yeasts transformed with AgMaT1/YEP112A1XE (plasmid YEP112A1XE containing the nucleic acid of AgMaT1) and grown on SC-glucose medium.

The black circles correspond to MaDH4 yeasts transformed with AgMaT1/YEP112A1XE (plasmid YEP112A1XE containing the nucleic acid of AgMaT1) and grown on SC-mannitol medium.

The curves represent the evolution according to the absorbance time (at 600 nm) of the yeast cultures. This increase of absorbance corresponds in fact to an increase of the number of yeasts in the culture medium and is representative of the growth rate of the yeasts. Therefore the yeasts transformed with the plasmids YEP112A1XE and AgMaT1/YEP112A1XE grow on glucose but only the yeasts transformed with the AgMaT1/YEP112A1XE plasmid are capable of growing on mannitol. It is therefore proof that AgMaT1 codes for a mannitol carrier.

FIG. 2 represents the absorption of mannitol in cells of S. cerevisiae. The external concentration of mannitol 3H is 500 μM and the pH is 4.5. The squares represent the absorption in transformed cells with the nucleic acid of AgMaT1 whilst the circles represent the absorption in control cells transformed with the empty YEP112A1XE plasmid. Only the transformed cells with the AgMaT1/YEP112A1XE plasmid are capable of absorbing the mannitol ³H placed in the external medium.

Material and Methods

Plant Material

Celery plants (Apium graveolens L. dulce variety, Vert d'Elne cultivar) were grown in greenhouses according to the conditions described by Davis et al. (1988). The phloemian bundles were isolated from adult petioles according to the technique described by Daie (1987).

Bacterial Strains and Yeasts.

The following strains were used in this study: Escherichia coli strains DH5α (supE44, ΔlacU169 (φ80, lacZM15), hsdR17, recA, endA1, gyrA96, thi-1, relA1) (strains commercially available from Clontech). XL1Blue MRF′ (Stratagene) and SOLR (Stratagene) were cultured according to standard techniques (Sambrook et al., 1989). The Saccharomyces cerevisiae MaDH4 strain (ura3, trp1LEU2, gap1-1, put4-1, uga4-1), the preparation of which is indicated hereafter, expresses the mannitol dehydrogenase gene of yeast and has been used for the functional characterization of the cDNA of AgMaT1. The 2a strain was obtained by crossing between the Δα (MATα, ura3, trp1, leu2) (Marcireau et al., 1992) and Σ22574d (MATα, ura3-1, gap1-1, put4-1, uga4-1) (Jauniaux et al., 1987) strains.

Expression Vector in Yeasts

The plasmid YIP 128A1, described in Riesmeier et al. (1992) is used. The mannitol dehydrogenase gene of yeast (YEL070) was amplified by PCR (polymerization chain reaction) using the oligonucleotides MDHPST5 (5-GACTCGA-GATGACAAAATCAGACGAAACAAC-3) and MDHBGL3 (5-GAAGATCTTCACACTTGGTCTAAAATTTCC-3) on the genomic DNA of the Saccharomyces Δα strain. The PCR product was cloned in the pBluescript SK vector digested beforehand by Pst1 and BamH1. After sequencing to confirm the sequence of the amplified gene, the PCR product was digested by Pst1 and Xba1 and cloned in the Pst1/Xba1 sites of YIP128A1. The construction was integrated into the genome of S. cerevisiae by the EcoV site in the leu2 gene in order to obtain the MaDH4 strain.

5′RACE-PCR (Rapid Amplification of the cDNA Ends by PCR),

The total RNA of celery leaves was isolated according to the method of Kay et al. (1987). The first cDNA strand was reverse transcribed from the total RNA with the degenerated primer (5′-CCNACNCC(G/A)AANGGNA(G/A)NA(G/A)-3) derived from the sequence LLGFGVG using reverse transcriptase SuperScript™ II (Stratagene). After degradation of the RNA matrix by RNaseH (Eurogentec), an anchoring primer (dC)₁₆ was created at the 3′ end of the single-stranded cDNA by a deoxynucleotydil transferase (GibcoBRL). A PCR amplification was carried out using the (dG)₁₆ and LLGFGVG primers under the following conditions: 2 minutes at 95° C. then 30 cycles comprising denaturation for 2 minutes at 95° C., fixation for 2 minutes at 55° C. and extension for 2 minutes at 72° C. The PCR products were analyzed by agarose gel electrophoresis then cloned in the pGEM-T Easy plasmid (Promega).

Construction and Screening of a cDNA Bank of Celery Phloem

The total RNA of the phloem bundles was isolated according to the method described by Kay et al. (1987). The polyA+ RNA was purified with the PolyATtract mRNA isolation system (Promega). A unidirectional EcOR1/XhoI bank was constructed in the Uni-ZapXR phage (Stratagene).

The recombinant phages (900,000) were screened with the radioactively labelled product of 5′RACE-PCR as probe, in accordance with the manufacturer's protocol (Stratagene). The Hybond TM-N nylon filters (Amersham) were hybridized overnight at 42° C. according to standard conditions (Stratagene). The filters were then rinsed for 15 minutes at 42° C. in SSC 2×(SSC 1×=0.15 M NaCl; 0.015 M sodium citrate) with 0.1% SDS, then for 15 minutes in the same medium but at 50° C. and 30 minutes at 50° C. in SSC 1× and 0.1% SDS. The excision in vivo was carried out on the 24 clones which produced a positive signal during the 3 successive screening turns. The identified cDNAs were partially sequenced. The sequence comparisons were carried out on the National Center for Biotechnology Information site. The transmembrane regions were predicted with the Tmpred program (Hofinann and Stoffel, 1993).

Expression of AgMaT1 in Saccharomyces Cerevisiae

The cDNA of AgMaT1 was ligated in the Pst1-XhoI sites of the yeast vector YEP112A1XE (Riesmeier et al., 1992). This vector allows the expression of the cDNA under the control of the yeast promoter ADH1. The MaDH4 yeast cells were rendered competent and transformed according to the protocol described by Dohmen et al. (1991).

Determination of the Growth Rate

The yeast cultures were grown on SC medium comprising either 2% glucose, or 2% mannitol. Aliquot fractions were taken regularly from the cultures and their absorbance was measured at 600 nm.

Determination of the Mannitol Dehydrogenase Activity

The cells were cultured until in logarithmic growth phase, rinsed in distilled water and resuspended at 80% (weight/volume) in extraction buffer (50 mM potassium phosphate pH 7.5, 1 mM DTT and 0.5% Triton X100). The cells were broken apart by vortex with glass beads. The cellular debris was eliminated by centrifuging and the crude extract used for the enzymatic assay. The mannitol dehydrogenase activity was measured at 30° C. according to Quain and Boulton (1987).

Measurement of the Transport of Radiolabelled Mannitol

The cells were cultured until the start of the logarithmic phase (corresponding to an absorbance of 0.6 to 600 nm), washed in distilled water and resuspended at 1% (weight/volume) in SC medium buffered to pH 4.5 with 25 mM MES. A 100 μl aliquot fraction of the cell suspension was incubated for 60, 120, 180 and 300 seconds in 100 μl of a solution containing of 500 μM [³H]-mannitol. The reaction was stopped by adding 8 ml of water at 4° C. and by filtration through glass fibre filters (Sartorius). The radioactivity incorporated in the yeast cells was determined by counting using liquid scintillation (Packard). For the experiments with inhibitors or competitors, the product was added 30 seconds before the radioactive mannitol.

Study of the Expression of AgMaT1 by RT-PCR (Reverse Transcription Followed by Polymerase Chain Amplification)

The total RNA of celery phloem was isolated according to the method of Kay et al. (1987). The first strand of cDNA was reverse transcribed from the total RNA with the oligo dT primer by using the reverse transcriptase SuperScript™ II (Stratagene). After degradation of the RNA matrix by RNaseH (Eurogentec), PCR amplification was carried out using the primers 5′ (ATTCTGGTGTGTTGCTCG) and 3′ (CAATGAACAGTATGATGTG) which allow the amplification of a fragment of 661 nucleotides. The PCR conditions were as follows: 2 minutes at 95° C. then 30 cycles comprising denaturation for 30 seconds at 95° C., fixation for one minute at 47° C. and extension for 45 seconds at 72° C. The PCR products were analyzed by agarose gel electrophoresis and the intensity of the signal obtained was quantified using Photoshop 5.0 software (Adobe systems Inc.). The extension factor e1F4A(10) (Mandel et al., 1995) was used as control gene, the expression of which is invariable.

Results

Molecular Cloning of AgMaT1

A certain number of proteins which transport sugars or metabolites show similarities in their sequences. It has been suggested that these transport proteins have evolved from the duplication of an ancestral protein with 6 transmembrane regions (Maiden et al., 1987). Several preserved amino acid regions were identified such as the amino acid sequences at the ends of the 6^(th) and 12^(th) transmembrane domains, PESPR and PETKG respectively (Griffith et al., 1992). Comparison between the different glucose carriers (MST1, STP1, STP4, HUP1, HUP3, GLUT1), the D-xylose carrier of L. brevis, the arabinose carrier of E. coli (ARAE), the galactose carrier of E. coli (GALP) and the myo-inositol carriers of yeast (genes ITR1 and ITR2) indicated a preserved region LLGFGVG. This sequence was chosen as matrix for designing the degenerated 5′RACE primer for PCR.

The first strand of cDNA was reverse transcribed from the entire RNA of mature celery leaves, primed with a degenerated primer LLGFGVG. After amplification, a band of 1 kb was observed on the agarose gel. All the fragments of this PCR reaction were cloned in a pGEM-T Easy vector (Promega), and several clones were obtained.

In order to obtain an entire clone, a cDNA library was constructed originating from phloem bundles isolated from mature celery petioles and this library was screened with the 5′RACE-PCR clone. After having screened 900,000 transformants, 24 positive clones were identified. The positive transformants with inserts of approximately 1.8-2.0 kb were chosen and partially sequenced. One of these clones, called AgMaT1, was chosen for detailed analysis. It contained 1778 pb with an open reading frame which codes for a protein containing 513 amino acids with a molecular mass estimated at 56 kDa. Hydropathic analysis of the deduced sequence of amino acids indicates that AgMaT1 contains 12 transmembrane domains and a long hydrophilic central region of 77 amino acid residues. The amino acid sequence of AgMaT1 was compared with those of the databases and it was found that this sequence was related to the sugar carriers in numerous organisms. The percentage identity of the amino acids is approximately equal to 50%. However, a greater percentage of identity (65%) was found with two optional sugar carriers of Beta vulgaris (Beet 1 and Beet 2). An asparagine residue, which is part of an N-glycosylation consensus sequence (Asn372), is situated on the external side and therefore must be glycosylated. In addition, the consensus sequences, which are the common characteristics of the sub-group of sugar carriers of MFS, are present in AgMaT1. The sequences of PESPRXL and PETQGRXXXE were found respectively at the ends of the 6^(th) and 12^(th) transmembrane domains, or the (R/K)XGR(R/K) motif between the 2^(nd) and the 3^(rd) and also the 8^(th) and 9^(th) transmembrane helices (Griffith et al., 1992).

Note:

The main difficulty encountered during cloning was the total absence of characterisation of such a carrier in any living organism. In fact the only mannitol carrier is a bacteria mannitol-phospho-transferase (Boer et al., 1994) which carries out both the transport and phosphorylation of mannitol. This combined system is present in bacteria for numerous substrates but it does not exist in Eukaryotic organisms. However, according to a first strategy, a first screening of the cDNA bank was carried out with the part of the gene of mannitol-phospho-transferase corresponding to the transmembrane field. This screening did not allow a result to be obtained, which is justified a posteriori by the absence of significant homology between AgMaT1 and the mannitol-phospho-transferase.

A second strategy, which turns out not to be operational, is inspired by that used for identifying the carrier of oligosaccharides in the plants (Patent EP 0,647,273). This consists of complementing the cells of Saccharomyces cerevisiae with a cDNA bank in an expression vector. The yeasts are in fact capable of using mannitol as a source of carbon, but they require a fairly long induction period on mannitol. As has already been specified, no mannitol carrier has been identified in yeast. The reasoning being that if a yeast expressed a plant mannitol carrier, this would confer on it a growth advantage and that therefore, it would grow quicker on a medium containing mannitol. The operation was carried out in this way but none of the cDNAs obtained showed any of the characteristics of membrane proteins and in fact resembled transcription factors. The selection system in fact allowed the cDNA which was involved in the expression of yeast genes to be identified and not the carriers.

Faced with the above difficulties, the Inventors formulated an improbable a priori hypothesis according to which the mannitol carrier would be part of the super family of glucide carriers described by Marger and Saier (1993). To do this, a species, celery, was used in which the existence of a mannitol carrier had been demonstrated (Salmon et al., 1995) and to construct a cDNA bank from the tissue (the phloem) in which the carrier was more expressed. The second stage was the selection of the cDNA obtained according to their capacity to confer the possibility of transporting mannitol to the yeasts. In these experiments the control was the strain of yeast transformed with the empty expression plasmid. In this way the mannitol carrier function of the cDNA of AgMaT1 was demonstrated.

During this experiment, other sequences were identified: in total 24 clones were obtained. Among all these clones, two were sequenced which showed the hydropathy profiles of carriers. The first, M22 (AgMaT1), conferred the ability to transport mannitol to the yeasts whilst the second, M7, did not confer it.

Construction of a Strain of Yeast Capable of Metabolizing Intracellular Mannitol

Initial studies were carried out in order to characterize the ability of a yeast to absorb and to metabolize mannitol (Quain and Boulton, 1987). Out of the 40 polyploid strains of S. cerevisiae screened, half of them have shown good growth on 5% mannitol after long-term adaptation (Quain and Boulton, 1987). As a result, it was decided to test different strains of yeasts for their ability to transport and metabolize the mannitol and 2 strains were retained. This was firstly carried out by analyzing the growth characteristics on a medium containing mannitol as the only carbon source. Σ22574d, generally deficient in a general carrier of amino acids and carrier of proline, is incapable of growth on a medium containing mannitol as the only carbon source. On the contrary, Δα was capable of growing on mannitol after long-term adaptation. After adaptation, the strain could be maintained successfully on a solid medium containing 5% mannitol. But maintenance of the adapted Δα strain on a solid medium only containing glucose leads to the total loss of the adapted growth. Such a growth adaptation on mannitol is probably due to the induction of the key degradation enzymes or the transport permeases. In accordance with the previous observations, NAD⁺ dependant D-mannitol dehydrogenase could be detected in the Δα yeasts (Table 1). TABLE 1 Activity of mannitol dehydrogenase in different yeast strains. The strains are developed in a liquid medium containing either 2% glucose or 2% mannitol. The results are the averages ± SD of the three independent experiments. ND, not detected. Activity (μmol of oxidized mannitol · (mg of protein)⁻¹ · min⁻¹) Strain glucose mannitol Δα 0.011 ± 0.003 0.240 ± 0.007 Σ22574d 0.006 ± 0.002 ND 2a 0.001 ± 0.001 ND MaDH4 0.410 ± 0.011 ND

In order to obtain an auxotrophy to tryptophan, the Δα strain (Trp) was crossed with the Σ22574d strain (Trp⁺). Yeast 2a was chosen, which cannot grow on a medium containing mannitol, with an auxotrophy to tryptophan and to leucine. No mannitol dehydrogenase activity was detected in cells 2a (Table 1). It was necessary to introduce a limited mannitol hydrolysis activity inside the yeast. The cDNA of the gene of the yeast mannitol dehydrogenase was cloned in YIP128A1 under the control of the ADH1 promoter and it was integrated in a stable manner in the leu2 gene of 2a. Several transformants have shown a mannitol dehydrogenase activity. The strain with the most significant activity, called MaDH4, was used for the subsequent analyses (Table 1).

Heterologous Expression of the AgMaT1 Protein

For a subsequent characterization of the function of the AgMaT1 protein, it was necessary to express the carrier in a functional manner in a heterologous system such as yeast cells. The cDNA of AgMaT1 was sub-cloned in the PstI/XhoI sites of the YEP 112A1XE shuttle vector which has a promoter/terminator box of the gene of alcohol dehydrogenase ADH1 of S. cerevisiae (Riesmeier et al., 1992). The competent MaDH4 cells were transformed with this construction and YEP 112A1XE was used as control.

All of the constructions were firstly tested for their ability to grow on mannitol as the only carbon source. As indicated in FIG. 1, the MaDH4 strain, transformed with the empty plasmid YEP 112A1XE is not capable of growing on mannitol. The cells expressing AgMaT1 could grow very well on this polyol. In order to directly test the ability of the transformed cells to transport mannitol, the yeast cells were incubated in a medium containing [³H]-mannitol for a few seconds to several minutes, the cells were washed and the radioactivity absorbed was measured by electric scintillation counting. FIG. 2 indicates that the transport of mannitol in the control cells of S. cerevisiae is negligible. However, the MaDH4 yeast strains, expressing AgMaT1, transport the [³H]-mannitol at high speeds when they grow on a medium containing mannitol. The same result is obtained with the cells of transformed yeast growing on glycerol (data not indicated).

Other polyols such as dulcitol, sorbitol, xylitol, myo-inositol appear capable of inhibiting by half the absorption of mannitol. The oside form of mannitol, mannose, appears to be recognized by AgMaT1.

Variation of the Expression of AgMaT1 During Saline Stress

The expression of AgMaT1 was monitored in plants having been subjected to saline stress for 4 weeks (daily watering with 300 mM of NaCl, Noiraud et al., 2000). The phloem of these plants as well as of the corresponding control plants (watered with water not containing NaCl) was removed in order to extract the RNA which was used to carry out RT-PCR reactions. If the expression of AgMaT1 in the phloem of the control plants is taken as base 100, the expression of AgMaT1 in the phloem of plants treated with NaCl is 500%, which represents a very significant stimulation and is in accordance with the role of AgMaT1 in saline stress tolerance in celery.

Transformation Protocol of Petioles or Leaves of Celery

Celery plants (approximately 10 cm in height) regenerated from embryogenic cells are used as plant material for the transformation.

Inoculation of the Celery Tissues

Agrobacterium tumefaciens bacteria are cultured for 24 hours at 28° C. under agitation in LB medium (Liquid Broth: 1% tryptone, 0.5% autolytic extract of yeast, 0.5% NaCl) with the appropriate antibiotic.

The petioles of celery plants are fragmented into sections of approximately 0.5 cm. For each fragment, a longitudinal section is produced. The celery segments are incubated in MS medium (Murashige & Skoog) 1×(normal concentration, i.e. no dilution) liquid containing {fraction (1/25)}^(th) of the culture of Agrobacterium tumefaciens bacteria for 60 minutes at ambient temperature.

Composition of the MS Medium Macro-elements CaCl₂  2.99 mM KH₂PO₄  1.25 mM KNO₃ 18.79 mM MgSO₄  1.50 mM NH₄NO₃ 20.61 mM

Vitamins Glycine 26.64 mM Myo-inositol 0.56 mM Nicotinic acid 4.06 μM Pyridoxine-HCl 2.43 μM Thiamine-HCl 0.30 μM

Micro-elements CoCl₂, 6 H₂O  0.11 μM CuSO₄, 5 H₂O  0.10 μM FeNaEDTA  0.10 μM H₃Bo₃  0.10 μM KI  5.00 μM MnSO₄, H₂O  0.10 mM Na₂MoO₄, 2 H₂O  1.03 μM ZnSO₄, 7 H₂O 29.91 μM The excess bacteria are then removed from the celery segments by arranging them on absorbent paper for 2-3 minutes.

Coculture of the Celery Segments and the A. Tumefaciens Bacteria

The cambial surface of the celery segments is left in contact with the gelosed regeneration medium RM. The Petri dishes are placed in a chamber air-conditioned at 25° C. for 48 hours and subjected to light/dark cycles of 16 hours/8 hours.

Elimination of the A. Tumefaciens Bacteria

After coculture for 48 hours, the celery segments are removed form the dishes of RM medium and transferred into MS 1 X liquid supplemented with cefotaxime at a final concentration of 250 μg/mL. After incubation for 60 minutes, the celery segments are dried on absorbent paper for 2-3 minutes.

Regeneration of Transformed Celery Plants

The cambial surface of the celery segments is left in contact with a gelosed callogenesis initiation medium CIM. The CIM Petri dishes are placed in a chamber air-conditioned at 25° C. and subjected to light/dark cycles of 16 hours/8 hours until the development of calluses (2-3 weeks). The celery segments are then transferred onto a gelosed organogenesis induction medium OIM (2-3 weeks). After the appearance of buds, these are removed and placed on gelosed rooting medium RM. A few weeks (3-4 weeks) are necessary for the development of young celery shoots.

Composition of the Media

Regeneration Medium RM MS 1× Mannitol 3.0% Saccharose 1.5% Casein hydrolysate 100.0 mg/L 6-Benzylaminopurine (BAP)  1.0 mg/L α-naphthylacetic acid (NAA)  0.1 mg/L Gibberellic acid (GA₃)  0.1 mg/L Agar 0.8%

Callogenesis Initiation Medium (CIM) MS 1× Mannitol 3.0% Saccharose 1.5% Casein hydrolysate 100.0 mg/L 6-Benzylaminopurine (BAP)  1.0 mg/L α-naphthylacetic acid (NAA)  0.1 mg/L Gibberellic acid (GA₃)  0.1 mg/L Kanamycin 125.0 mg/L Cefotaxime 200.0 mg/L Agar 0.8%

Organogenesis Induction Medium (OIM) MS 1× Mannitol 3.0% Saccharose 1.5% Casein hydrolysate 100.0 mg/L 6-Benzylaminopurine (BAP)  1.0 mg/L Gibberellic acid (GA₃)  0.1 mg/L Kanamycin  75.0 mg/L Cefotaxime 200.0 mg/L Agar 0.8%

Rooting Medium (RM) MS 1× Mannitol 3.0% Saccharose 1.5% Casein hydrolysate 100.0 mg/L α-indolyacetic acid (IAA)  0.1 mg/L Kanamycin  75.0 mg/L Cefotaxime 200.0 mg/L Agar 0.8%

REFERENCES

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1. Use of a DNA sequence coding for a linear polyol carrier, in plants and fungi, such as polyols having main chain containing 5 to 8 carbon atoms, in particular 5 to 7 carbon atoms, in particular 6 carbon atoms, these polyols being advantageously chosen from mannitol, sorbitol, dulcitol, galactitol, inositol, myo-inositol, ribitol and xylitol, and being in particular mannitol, for the preparation of transgenic plants.
 2. Use according to claim 1, in which the DNA sequence is chosen from one of the following sequences: SEQ ID NO: 1, SEQ ID NO: 3, SEQ ID NO: 4, SEQ ID NO: 5, SEQ ID NO: 6, SEQ ID NO: 7, SEQ ID NO: 8, SEQ ID NO: 9 and SEQ ID NO:
 10. 3. Protein characterized in that it comprises or is constituted by: sequence SEQ ID NO: 2, or any sequence derived from SEQ ID NO: 2, in particular by substitution, suppression or addition of one or more amino acids, having the property of transporting linear polyols in plants and fungi, such as polyols having a main chain containing 5 to 8 carbon atoms, in particular 5 to 7 carbon atoms, in particular 6 carbon atoms, these polyols being advantageously chosen from mannitol, sorbitol, dulcitol, galactitol, inositol, ribitol and xylitol, and being in particular mannitol, any homologous sequence of SEQ ID NO: 2, preferably having a homology of at least approximately 50% with sequence SEQ ID NO: 2 and possessing the property of transporting, in plants and fungi, polyols as defined above, or any fragment of one of the sequences defined above, on the condition that it possesses the property of transporting, in plants and fungi, polyols as defined above, in particular any fragment being constituted by at least approximately 10 amino acids adjacent in the sequence SEQ ID NO:
 2. 4. Nucleotide sequence coding for a protein as defined in claim
 3. 5. DNA sequence which comprises or is constituted by: nucleotide sequence SEQ ID NO: 1, or any nucleotide sequence derived, by degeneration of the genetic code, from the sequence SEQ ID NO: 1 coding for a protein represented by SEQ ID NO: 2, or any nucleotide sequence derived, in particular by substitution, suppression or addition of one or more nucleotides, from the sequence SEQ ID NO: 1 coding for a protein derived from SEQ ID NO: 2, as defined in claim 3, or any homologous nucleotide sequence of SEQ ID NO: 1, preferably having a homology of at least approximately 35% with the sequence SEQ ID NO: 1 coding for a homologous protein of SEQ ID NO: 2, as defined in claim 3, or any fragment of the nucleotide sequence SEQ ID NO: 1 or of the nucleotide sequences defined above, said fragment being preferably constituted of at least approximately 30 nucleotides adjacent in said sequence, or any complementary nucleotide sequence of the above-mentioned sequences or fragments, or any nucleotide sequence capable of hybridizing in stringent conditions with the complementary sequence of one of the abovementioned sequences fragments.
 6. Recombinant vector, in particular plasmid, cosmid, phage or virus DNA, containing a nucleotide sequence according to any one of claims 4 or
 5. 7. Recombinant vector according to claim 6, containing the elements necessary for the expression in a host cell of polypeptides coded by the nucleic acids according to one of claims 4 or 5, inserted into said vector.
 8. Host cell, chosen in particular from bacteria, viruses, yeasts, fingi, plants or mammal cells, said host cell being transformed, in particular using a recombinant vector according to any one of claims 6 or
 7. 9. Antisense oligonucleotides or antisense messenger RNA derivatives of the nucleotide sequences according to one of claims 4 or
 5. 10. Plant cells containing in their genome a nucleotide sequence according to one of claims 4 or
 5. 11. Transgenic plants, parts of plants, plant seeds or plant propagation material containing cells according to claim
 10. 12. Transgenic plant according to claim 11, which, in its native state, does not contain or express the gene of the mannitol carrier, into the genome of which is introduced a nucleotide sequence according to one of claims 4 or
 5. 13. Transgenic plant according to claim 11, which, in its native state, contains or expresses the gene of the mannitol carrier, into the genome of which is introduced a nucleotide sequence according to one of claims 4 or
 5. 14. Process of screening genetically modified plants with at least one nucleotide sequence of interest which comprises the following stages: the transformation of plant cells with a vector containing an insertion sequence, said insertion sequence comprising the nucleotide sequence of interest and a nucleotide sequence coding for a polyol carrier as defined in one of claims 1, 2, 4 and 5, the culture of the cells thus transformed on a medium containing said polyol as the only source of carbon, to obtain transgenic plants or fragments of transgenic plants containing said insertion sequence.
 15. Process for obtaining transgenic plants resistant to pathogens, which comprises the following stages: the transformation of plant cells with a nucleotide sequence coding for a polyol carrier as defined in one of claims 1, 2, 4 and 5, the culture of the cells thus transformed to obtain transgenic plants or fragments of transgenic plants.
 16. Process for obtaining transgenic plants resistant to saline stress, which comprises the following stages: the transformation of plant cells with a nucleotide sequence coding for a polyol carrier as defined in one of claims 1, 2, 4 and 5, the culture of the cells thus transformed to obtain transgenic plants or fragments of transgenic plants. 